One important function of a mobile radio network is mobility support, i.e., the ability to maintain a mobile radio connection of a mobile station (MS) even when the latter is moving out of the receiving range of one base station (BS) into the receiving range of another base station. For this purpose a so-called handover (HO) is initiated, i.e., the connection of a mobile station to a base station A is handed over to a base station B at a defined point of time.
In the context of a handover there are mainly two types of errors: On the one hand, there are unnecessary handovers (so-called ping-pong effects), i.e., a multiple and unnecessary switchover of a mobile station between two or more base stations. Ping-pong effects lead to an unnecessary exchange of management messages and should therefore be avoided. One method for detecting these effects is described for example in DE-A-10 2007 038 099. Another type of error is constituted by erroneous handovers (or so-called handover errors) which can result in a loss of the connection to a mobile station. These errors might for example occur when a handover is initiated too early or too late so that, at the time of the handover, the radio coverage by base station A or base station B cannot be ensured. Handover errors in any case lead to an interruption of the connection with a mobile station, but can also lead to a complete loss of the connection in extreme cases, and should therefore be avoided. The detection of handover errors has been known in the state of the art for some time.
The decision whether to initiate a handover, and also the moment of the handover, is dependent on several parameters, the so-called handover parameters. Configuration values for these parameters are stored in the mobile radio network (in networks according to the LTE standard, for example, in each base station, in networks according to the UMTS standard in a superordinate network component, the radio network controller—RNC). Widely used configuration parameters are the so-called hysteresis margin or the so-called time to trigger. The hysteresis margin (or hysteresis threshold) expresses how much stronger the transmission power received by the mobile station or the field strength of the target base station (corresponding to base station B mentioned above) has to be compared to the transmission power received by the source base station (corresponding to base station A) before a handover is initiated (see for example DE-A-10 2007 038 099). The time to trigger expresses for how long (e.g. how many milliseconds) the received transmission power of the target base station at the mobile station must be larger by the hysteresis margin as the received transmission power of the source base station before a handover is initiated.
There can be a number of further parameters in addition to these exemplary parameters, which have been listed because they occur in most mobile radio systems.
The settings of these configuration parameters are decisive for the initiation of handovers in a mobile radio network and therefore also have an impact on the number of ping-pong effects and handover errors that will occur. For this reason, in conventional networks, the statistics on ping-pong effects and handover errors are evaluated in regular intervals and the parameter settings are optimized in case of abnormally high values. This has some major disadvantages. First, a high proportion of manual processes entails high costs. In addition it might be sensible to modify the configuration values of the handover parameters on short notice if network conditions change (e.g. traffic jams on the motorway resulting in a temporarily reduced speed of the mobile station). This is nearly impossible in the state of the art.
To overcome these disadvantages, so-called self-optimization processes are the subject matter of current research (see for example the EU-funded research program Self-Optimisation & Self-Configuration in wireless networks (SOCRATES): Self-optimisation in future mobile access networks, Presentation at Mobile Network Optimisation 2008, Cannes, France, Nov. 3-6, 2008). With respect to handover parameters this means that it would be desirable for the network to be able to optimize the configuration values of the parameters autonomously. Initial approaches in this direction can be found, e.g., in DE-A-10 2007 038 099 (Paragraph [0012]: “to develop the methods for avoiding ping-pong handover in mobile radio networks in a way that the parameters, particularly the hysteresis of the reception field strength . . . can be automatically and autonomously set in order to avoid ping-pong handover . . . ”) and US-A-2004/066434 (here, the configuration value of the “hysteresis” parameter changes between two fixed values).